This invention relates to lighting control systems and more particularly, to a lighting control system which automatically maintains high quality illumination while optimizing the conservation of energy.
Most lighting systems are designed without regard for the benefits of natural daylighting. Daylighting is an illumination source which can be utilized to achieve better conservation of energy through appropriate building design. One recent office building design, for example, uses up to 80 percent natural daylighting. Because the building is also used when it is dark outside, such as on early winter mornings, the building's lighting system must be able to provide illumination over a range of 20 percent artificial lighting (80 percent daylighting) to 100 percent artificial lighting (no daylighting). The only reliable system that can adapt to such a wide range of artificial lighting levels and also optimize the conservation of energy is one which is entirely automatic.
In addition, it is generally recognized that compared to illumination from ceiling mounted electric lighting, an equal amount of daylight illumination from windows can be about three times more effective in producing visibility. Thus, visibility-based lighting controls will result in greater energy conservation than will illumination-based lighting controls. However, presently available automatic dimming systems which adjust for daylighting are photocell controlled. These photocells only respond to raw footcandles which are not directly related to the visibility provided by the illumination sources. The raw footcandle data must be correlated to a criteria for quality task illumination in order to provide a measure of visibility.